Conventional globe style control valves and other types of valves commonly include screwed-in or clamped-in seat rings. Seat rings are typically inserted into a cylindrical cavity formed within a valve body of a valve and include an annular thrust-bearing rim against which a valve plug may be applied to close the valve. The seat ring is subject to wear and, therefore, is typically formed as a removable part to facilitate replacement. Several economical materials can be used to construct seat rings, including steel, stainless steel, and hardened materials such as stellited, ceramic, and Tungsten carbide. The clamped-in seat ring typically requires a compressed gasket to seal the seat ring and prevent fluid leakage. The clamped construction typically requires additional parts and complexity due to the load that is needed in the valve assembly to compress the gasket. Screwed-in seat rings, on the other hand, generally do not require a gasket, but rely on a metal-to-metal sealing formed by complementary surfaces formed in the seat ring and the valve body. However, conventional screwed-in seat rings suffer certain alignment disadvantages, concentricity disadvantages, and seal limitations as described below.
FIG. 1 is a cross-sectional view of prior art seat ring valve assembly 10, which includes seat ring 100 screwed into an interior cylindrical sidewall surface 122 of valve body 120. Seat ring 100 includes annular thrust-bearing rim 102, which engages a valve plug 160 to close the valve. When in the closed position, the valve plug 160 is compressed against the seat ring 100 and prevents fluid from flowing through a passage 152 created when the valve plug is lifted from the seat ring. The seat ring 100 further includes an exterior cylindrical sidewall 112, which is generally formed at a 90-degree angle to an upper flange 113 and faces an interior cylindrical surface 138 of the valve body 120 (described in more detail below). Just below the cylindrical sidewall 112, a tapered exterior surface 132 extends generally downward and toward a center of the seat ring 100. The cylindrical sidewall 112 is sloped to complement a tapered valve body surface 136 of the valve body 120. Both tapered surfaces, 132 and 136, are typically manufactured to be oriented at approximately 45 degrees with respect to a center axis 137 along which the plug actuates. Engagement of the tapered surfaces, 132 and 136, forms a primary seal 106 between the seat ring 100 and the valve body 120. Below this sealing surface, a first threaded portion 104 of the seat ring 100 mates with a second threaded portion 110 of the valve body 120, which helps secure and guide the seat ring 100 into the cavity 130. The threaded engagement also provides a secondary seal.
The effectiveness of primary seal 106, however, is highly dependent on the alignment (meaning straightness and centering) of the seat ring 100 within the cavity 130. If the seat ring 100 is misaligned, a tight uniform circular seal will not occur because certain sections of the primary seal 106 will have gaps, particularly where there are minute surface flaws in the metal, resulting in unacceptable leakage between the seat ring 100 and the valve body 120. Additionally, the effectiveness of the seal formed by the interface of the thrust-bearing rim 102 and the valve plug 160 is also highly dependent on the alignment of the seat ring 100 within the cavity 130. In this case, if the seat ring 100 is misaligned with respect to the center axis 137, then the thrust-bearing rim 102 is correspondingly misaligned with respect to the center axis. Therefore, as the valve plug 160 closes, it will fail to form a tight circular seal with the thrust-bearing rim 102. Unfortunately, conventional screwed-in seat ring assemblies of this type are particularly susceptible to misalignment resulting in leakage for at least three reasons. First, conventional screwed-in seat rings rely on their mated threads to provide alignment and a secondary seal. However, industry standard threads typically include relatively loose tolerance requirements, resulting in excessive clearance or play between the mating threaded parts. Conventional valve seat assemblies use the threaded connection between the seat ring 100 and valve body 120 to position the seat ring, and therefore the ultimate location of the thrust-bearing rim 102 may vary. The valve plug 160 is movably positioned with respect to the valve body 120, and therefore the imprecise location of the seat ring 100 within the valve body cavity 130 increases the uncertainty that the seat ring 100 and plug 160 will be concentric. As used herein, the terms “concentric” and “concentricity” mean that a center axis of the seat ring 100 and the center axis 137 are substantially aligned. Both the valve plug 160 and the valve body 120 are concentric with respect to the center axis, thus a lack of thrust-bearing rim 102 concentricity may result in misalignment of the valve plug 160 causing leakage. Second, the area in which the sidewall 112 joins the tapered exterior surface 132 of the seat ring 100 is formed as a sharp edge that may contact the valve body surface 136 during assembly. As such, minute surface imperfections or irregularities in this edge area may result in significant misalignment, therefore small machining tolerances are required. Third, the relatively shallow angle (45 degrees) of the valve body surface 136 does not sufficiently direct the seat ring 100 toward the center of the cavity 130, causing the seat ring 100 to be susceptible to misalignment. In other words, a conventional seat ring is particularly susceptible to leakage because it is either not properly centered, misaligned, or both, causing at least a portion of the seal 106 to be susceptible to leakage where there is insufficient contact between mating surfaces.
The effectiveness of the primary seal 106 is also highly dependent on tolerances associated with the tapered surfaces, 132 and 136. Normal industry standards introduce tolerance differences between the tapered valve body surface 136 which prevent true parallel sealing surfaces. Such tolerance differences prevent a tight uniform seal resulting in unacceptable leakage between the seat ring 100 and the valve body 120. Additionally, the aforementioned problems caused by thread clearance are further exacerbated by such tolerance differences.
Alternatively, in light of the difficulty of controlling tolerances when manufacturing parallel surfaces, intentional angular deviations between the sealing surfaces present similar problems. In such a configuration, the sharp edge of the sidewall surface 112 tends to engage and impinge (i.e., dig into) to the tapered surface 136, which may also cause misalignment between the thrust-bearing rim 102 and the valve plug 160. To compensate for these limitations and to reduce leakage to an acceptable level, the assembler must apply an unacceptably high level of torque to the seat ring 100 to excessively compress it within the cavity 130. The required excessive compression creates a complicated assembly process, causes torque stress on the assembly parts, and contributes to a high failure rate of the primary seal 106.
What is needed is an improved seat ring valve assembly having self-aligning characteristics for creating a stronger seal with reduced leakage without requiring an unacceptably high level of torque during assembly.
While the disclosure is susceptible to various modifications and alternative constructions, certain illustrative embodiments thereof are shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the disclosure to the specific embodiments disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the disclosure as defined by the appended claims.